1. Field of the Invention
This invention relates broadly to prostheses. More particularly, this invention relates to prostheses for the total or partial replacement of ossicles in the middle ear.
2. State of the Art
Hearing is facilitated by the tympanic membrane transforming sound in the form of acoustic sound waves within the outer ear into mechanical vibrations through the chain of ossicular bones (malleus, incus, stapes) in the middle ear. These vibrations are transmitted through the ossicular bones to the footplate of the stapes where micro or macro motion of this structure results in compression waves within the fluid of the inner ear. These compression waves lead to vibrations of the cilia (hair cells) located within the cochlear where they are translated into nerve impulses. The nerve impulses are sent to the brain via the cochlear nerve and are interpreted in the brain as sound.
Hearing efficiency can be lost to erosion of the ossicular bones. Various combinations or portions of the bones can be replaced. For example, all of the ossicles between the tympanic membrane and the stapes footplates can be replaced using a total ossicular replacement prosthesis, or TORP. Alternatively, the malleus and incus can be replaced leaving all or a portion of the stapes intact. The prosthesis for such a procedure is a partial ossicular replacement prosthesis, or PORP.
Depending on the ossicular replacement, various different configurations of prostheses can be used. For example, a TORP generally extends from the tympanic membrane to the footplate of the stapes, and distributes force from its head end at the tympanic membrane to its distal end (shoe) positioned on the footplate. A PORP generally extends from the tympanic membrane to the capitulum and/or junction of the crura of the stapes. The proximal end of the PORP includes a head that distributes force across the tympanic membrane and the distal end includes a bell or cup that seats over the capitulum and crura of the stapes.
For each type of ossicular prosthesis, several lengths must be provided given the natural differences in anatomical distances between middle ear structures in different patients. This requires that a device company manufacture, and that a surgeon (or medical facility) inventory, various sized prosthesis to accommodate the variations in dimensions across the anatomy of patients.
Moreover, due to ambient or dynamic changes in pressure within the middle ear after implantation, e.g., by sneezing or high sound pressure levels (SPL) caused by an intense noise, the distance between prosthesis coupling points can change. This may situation may result in dislodgement of the prosthesis or otherwise lead to poor sound conduction along the ossicular chain. Further, post-operative scarring down can lead to the implanted device being too long, possibly resulting in a negative effective on sound conduction. Spring elements have been considered to accommodate the change in distance that occurs during pressure changes. Bornitz, Design Considerations for Length Variable Prostheses Finite Element Model Simulations, Middle Ear Mechanics in Research and Otology: 153-160 (2004), states that good sound conduction is provided by prostheses with stiff springs, but that such springs provide only very small amounts of compression (≦0.02 mm under a static load of 5 mN), which is insufficient to accommodate the change in distance under pressure. Bornitz also determined that a soft spring can provide a suitable change in compression (up to 0.53 mm under a static load of 5 mN force), but has unacceptably poor sound transfer characteristics.